Daniell equilibrium cell voltage

It was mentioned earlier that the equilibrium cell voltage A%, is equal to the difference between the equilibrium potentials of its half-cells e.g., for the Daniell element,... 

The question now arises as to what factors are responsible for determining the rates at which the various cell processes occur. Thermodynamic arguments permit the feasibility of overall cell reactions to be predicted, but give no information on rates. To understand the latter it is necessary to consider the effects on various parts of the cell of forcing the cell voltage to assume a value different from that of the equilibrium emf. It has been shown above that in the Daniell cell at equilibrium, charge transfer across the zinc/solution interface can be described in terms of processes... 

The voltage of a single metal electrode in the half-cell of an electrochemical cell, such as the copper and zinc electrodes of the Daniell cell, cannot be measured. If the metal electrode is connected to a voltmeter using a wire and the wire placed in the solution to complete the circuit, another redox equilibrium and electrode potential will be generated. The voltage will be the difference of the two electrode potentials, not the voltage of the first metal electrode in equilibrium with its ions in aqueous solution. 

If the concentration of zinc ions in the half-cell of the Daniell cell is increased, then the equilibrium is shifted to the right and the negative charge on the electrode is decreased the addition of zinc ions will cause some of the zinc ions to gain electrons. This will decrease the voltage of the Daniell cell to a value below 1.10 V. 

The back e.m.f. is a voltage that opposes the passage of a current through an electrolytic cell. There are three sources of the back e.m.f. The first is the reversible back e.m.f. due to the cell reaction. For example, in a Daniell cell with unit activities the reversible back e.m.f. is the equilibrium standard-state cell potential of 1.100 V. For activities other than unit activities, the reversible back e.m.f. can be calculated from the Nernst equation. For an infinitesimal electrolytic current, the reversible back e.m.f. is the only contribution to the back e.m.f. For a finite current, the IR drop in the voltage across the electrolyte solution due to its electrical resistance also contributes. In many cases, we will be able to neglect this contribution. The third source of back e.m.f. for a finite current is the overpotential, which is due to the polarization of the electrode. 

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